I'm playing through campaign a 2nd time and found these two weapons at the top of the spiral staircase on level 10. The Annihilator Greatsword is wielded by an armored enemy at the very top so make sure you don't knock him off the stairwell when killing him. Also, you can find the Decapitator Battle Axe by going to the very top where the spiral stairwell breaks off, look to your right, and you will see it on a wooden beam. It's in plain sight, but I missed it first playthrough.

*Also, this is more well known, but at the end of level 9 before burning the barricade and entering the tower, look up and you'll see a barrel on a ledge. Shoot it down for the Spellbinder Battle Axe.
*Another side note: pickup the final boss's hammer when he's downed